In the pulping of comminuted cellulosic fibrous material, preferably but not excluded to wood chips, in a continuous digester the material is first treated to remove air bound in the cellulosic fibrous material. Typically, the cellulosic fibrous material is steamed to remove the material of air while simultaneously increasing the temperature to about 80-100° C. The steaming process will normally release the natural acidity of the wood material and the pH value in any drained steam condensate could easily reach 4-5. The steamed cellulosic fibrous material is thereafter slurried or impregnated in an impregnation or slurrying liquid with sufficient amount of chemicals, i.e. alkali and sulfidity in case of a kraft process.
The slurried cellulosic fibrous material is transported as slurry to the pressurized digester or impregnation vessel using high pressure pumps or a high pressure sluice feeder, and with a top separator arranged in the top of the pressurized impregnation vessel or in the top of the digester. The typical digester pressure is more than 5 bar (>0.5 MPa).
In conventional systems these steaming and slurrying systems have been installed as a system preceding the pressurized impregnation vessel or the pressurized digester vessel. The systems preceding the pressurized vessel have included expensive and energy consuming machines.
For a typical digester system, following systems and machines have been used;                Chip bins,        Steaming vessels        Slurrying chutes        High pressure sluice feeder and/or high pressure pumps        Impregnation vessels        
Only to transport the slurried chips to the pressurized impregnation or digester vessel requires some 400 kW per ADT pulp produced. In a digester with a capacity of some 5000 ADT per day is thus required and a pumping system with an installed power available in the order of some 2 MW.
These systems and associated equipment and building structure are a large part of the total investment costs of a continuous digester system. Also, the operating costs of these systems and machines take a large part of the production costs for the pulp produced.
U.S. Pat. No. 3,303,088 disclosed already in mid 1960-ties a process using a single hydraulic digester, but with separate chip bin, steaming vessel, slurring tank and high pressure pumps ahead of the single hydraulic digester.
U.S. Pat. No. 5,635,025 disclosed an effort to patent the concept of a single vessel for the entire pre-treatment of chips, including the functions of a chip bin, a steaming vessel and the chip chute. This single pre-treatment vessel was located ahead of the transfer system including the high pressure sluice feeder. The corresponding Swedish application was abandoned as the concept with a common chip bin, steaming vessel and chip chute was anticipated by U.S. Pat. No. 3,532,594 from the mid 1960-ties.
A further improvement of the pre-treatment systems in a single impregnation vessel is disclosed in U.S. Pat. No. 7,381,302, where the impregnation vessel is held substantially at atmospheric pressure, and impregnation liquids at successively higher temperatures where added at successively increasing depth in the liquid volume established in the impregnation vessel. Still, the conventional high pressure sluice feeder was located after this impregnation vessel for feeding the impregnated chips to the pressurized digester. This type of atmospheric impregnation vessel, called the IMPBIN™ system guarantees that the chips are both steamed and impregnated at low temperature, resulting in easy cooking at low reject volumes and high pulp quality. The IMPBIN concept has been installed in a number of new digester systems throughout the world, in mills having capacities in the order of 3000-6000 ADT per day and has proven to be a success. One further advantage with the IMPBIN system is that this could be operated with “cold top” control, i.e. avoiding blow trough of steam, which reduce energy losses in gas handling systems needed as the amount of hot gases driven off from the chips and needing condensation is dramatically reduced.
The fait of the IMPBIN™ system has been challenged as the conventional approach has been using excessive steaming systems in chip bins and steaming vessels, and this excessive steaming has been perceived as a necessity in order to purge all air from chips and be able to establish a correct column movement of the chips in the digester.
However, excessive steaming in pre-treatment establish a high chip temperature and in subsequent impregnation stages is the cooking chemicals consumed as they penetrate the chips, preventing cooking chemicals from penetrating into the core of the chips and as a consequence causing high reject volumes.
The IMPBIN™ system has in spite of this proven to be fully sufficient in establishing the necessary impregnation of the chips and a smooth column movement inside the digester.
The present invention is related to a further improvement and simplification of the digester system, where both the installation costs, i.e. investment costs, and operating costs are dramatically reduced.
In view of the success of the IMPBIN™ system, this general impregnation concept could be integrated with the actual digester, and a true “single vessel” digester system would be obtained. By this integration are several major advantages obtained, such as;                No need to classify the digester vessel as a pressure vessel; and        Guaranteed low temperature impregnation, and        No power losses in chip transfer to a pressurized digester; and        No high pressure transfer systems, and        No expensive top separator mounted at the top of the digester; and        No need for chip bins, steaming vessels, and chip chutes etc.        
In following parts are an atmospheric vessel referred to, and this implies a vessel not qualified as a pressure vessel and associated required testing and certification for a pressure vessel. According to European legislation a vessel must be classified as a pressure vessel if the pressure applied in the vessel is exceeding 0.5 bar. Thus, the atmospheric vessel could thus have a pressure established in the top substantially at atmospheric pressure, i.e. 0 bar (g), or a slight positive pressure of up to 0.5 bar (g) or slight negative pressure of down to −0.5 bar (g). The small deviation from a perfect atmospheric pressure is most often wanted for a controlled venting of the atmospheric phase in the top of the vessel as air may enter into the vessel with the raw material, i.e. chips, and a small leakage flow of malodorous gases could escape from the underlying chip volume. Preferably only an incremental positive pressure or negative pressure in the order of 0.1-0.2 bar is implemented, but still qualifying the vessel as an atmospheric vessel. The actual pressure established is controlled by the venting system, and parallel safety valves in form of reliable water-locks.
The establishment of a single vertically oriented atmospheric vessel enables a successive implementation of hotter treatment zones throughout the digester, and no need for a pressurized digester vessel is at hand, nor any separate pre-treatment systems, nor any high pressure transfer devices. The principle applied is similar to that one shown for the impregnation vessel IMPBIN™ as shown in U.S. Pat. No. 7,381,302, but now applied to the entire cooking process. The possible temperature profiling throughout the vessel is given by following table;
TLIQSat. PΔHatmΔH+0.5ΔH−0.5(° C.)(kPa)(meter)(meter)(meter)105120.8>2  —>7  110143.3>4.3—>9.3115169.1>6.9>1.9>11.9 120198.5>9.8>4.8>14.8 125232.1>13.2 >8.2>18.2 130270.1>17.0 >12   >23   135313.0>23.3 >18.3 >28.3 140361.3>26.1 >21.1 >31.1 145415.4>31.5 >26.5 Where;TLIQ is the possible temperature of the liquid in vesselSat. P is the saturation pressure at the actual temperatureΔHatm/ΔH+0.5/ ΔH−0.5 are minimum depths under liquid level at atmospheric/+0.5 bar/−0.5 bar pressures in vessel top.
According to the present invention a continuous digester system is used that has only a single generally vertically oriented atmospheric vessel having a top and a bottom for receiving comminuted cellulose fibrous raw material and within the vessel steaming, slurrying, impregnating and digesting the fibrous material before feeding out digested fibrous material from the bottom of the vessel.
In the inlet of the vessel is any suitable metering means installed for continuously feeding the fibrous raw material into the vessel from the top thereof. The metering means could be a conventional chip meter having a rotor with pockets of a predefined volume.
The vessel also has means for establishing a first level of fibrous raw material in the vessel. This level could be monitored by any suitable conventional chip level meter available in the field.
In order to control the atmospheric pressure in the top of the vessel also the vessel has means for establishing a pressure in the top of the vessel at substantially atmospheric pressure in the range of +0.5 to −0.5 bar (g). The vessel also has means for establishing a second level of liquid in the vessel. The second level is below the first level thus creating a fibrous raw material volume in a pile above a total liquid volume in the vessel.
This pile of raw material volume provides a triple function, as                condensation surfaces for any steam penetrating upwards, and        a location for steaming action from underlying hotter liquids, purging air from chips, and        a thrust force for the chips downward into the liquid volume.        
The vessel also includes means for supplying impregnation liquids to a first end of a first upper volume of liquid in the total liquid volume held by the vessel, and also means for supplying cooking liquids to a first end of a second lower volume of liquid in the total liquid volume held by the vessel.
For heating to cooking temperature the vessel also has means for heating at least the cooking liquids in the second lower volume of liquid in the total liquid volume held by the vessel.
The first upper volume of liquid containing the impregnation zone has preferably a height of at least 17 meters, and preferably in the range of 17-40 meters, and more preferably in the range of 20-30 meters, which will enable typical cooking temperatures in the subsequent second lower volume of liquid containing the cooking zone.
The second lower volume of liquid containing the cooking zone has preferably a height of at least 30 meters, and more preferably at least 40-50 meters, which will enable sufficient retention time in the cooking zone at normal cooking temperatures, resulting in the required H-factor for successful delignification process.
The total height of the vessel, containing the impregnation and cooking zones is thus preferably at least 70 meters high, and preferably in the range of 75-90 meters, but should not result in a total height of liquid in the vessel exceeding 100 meters or a height of comminuted cellulose fibrous raw material exceeding 120 meters, as to high chip column may impede operation of the digester circulations due to compacting effects in the bottom of the digester. The total height should more preferably be 75-90 meters, but should not result in a total height of liquid in the vessel exceeding 100 meters or a height of comminuted cellulose fibrous raw material exceeding 120 meters. The required heights of liquids are controlled by controlling the net liquid flows entering and leaving the vessel in a conventional manner.
The vessel also has means for withdrawing spent cooking liquid from the end of the second lower volume of liquid. The vessel preferably also includes a final zone for cooling and washing the processed material. Finally, the vessel has means for continuously withdrawing slurry of digested fibrous raw material from adjacent the bottom of the vessel and feeding the slurry to subsequent post cooking systems.
Typically the digested fibrous raw material is sent to post cooking systems such as brown washing, screening, mechanical refining or any chemical pre-bleaching stages such as oxygen delignification, ozone bleaching or similar first pre-bleaching stages, all depending on the subsequent use of the digested pulp.
According to the present invention now described will the atmospheric vessel be the only handling vessel where the fibrous raw material is purged from air, impregnated and digested to an extent that the digested fibrous raw material is delignified and reaching a kappa number below 120.
High yield pulp typically used for liner is digested to a kappa number in the order of 60-90, but other pulps used for bleached grades of paper are typically digested to a kappa number in the order of 15-30.
In a preferred embodiment, the present invention has the means for heating the cooking liquids comprising a first liquid circulation conduit having a screen in the wall of the vessel in first end of the circulation conduit and an outlet pipe in the centre of the vessel at the second end of the circulation conduit, and a pump in the circulation conduit, wherein the liquid in the circulation conduit is passing a heater for heating the liquid circulated in the circulation conduit and wherein the first and second end of the first circulation conduit is located in the second lower volume of liquid.
In the most simplified form of the present invention all or the overwhelming part of the heating could be made to the cooking stage, and preceding stages could be heated by sending hot liquids from cooking stage in counter current flow upwards in the vessel. Either in a displacement function, where the hotter liquid is displacing the colder liquid, or using the heat in the liquids in heat exchangers.
In a further preferred embodiment of the present invention the means for supplying cooking liquids, preferably in form of white liquor, has a second liquid circulation conduit having a screen in the wall of the vessel in first end of the circulation conduit and an outlet pipe in the center of the vessel at the second end of the circulation conduit, and a pump in the circulation conduit, wherein the liquid in the circulation conduit receives fresh cooking chemicals to the liquid circulated in the circulation conduit and wherein the first and second end of the second circulation conduit are located in the second lower volume of liquid. Alternatively cooking liquids could be used such as white liquor, kraft black liquor, green liquor, or sulfite cooking liquor.
In the simplest embodiment of the present invention the first and second liquid circulation conduits used for heating and supplying cooking chemicals, could be one and the same liquid circulation conduit.
The means for heating the cooking liquids includes preferably a heater in the form of an indirect heat exchanger, where the heating medium used is steam. Indirect heating is preferred as the clean condensate obtained from any such indirect heaters could be used again in the clean steam production systems, and further dilution of cooking liquors with water is avoided.
In a yet a further preferred embodiment, the present invention has means for supplying impregnation liquids using as a liquid source at least partly a liquid withdrawn from the cooking zone in the second lower volume of liquid. Preferably a semi-spent cooking liquor is used, which still has a relatively high residual alkali content, well over 6 g/l and typically in the range of 6-12 g/l. Such semi-spent cooking liquor is also typically having a high sulfidity level which is advantageous for the impregnation process. The means for supplying impregnation liquids could also use as liquid source at least partly fresh cooking chemicals, preferably white liquor. This additional charge of fresh cooking liquors could be made to establish a sufficient neutralization of the wood acidity released from the original raw material, and establishment of sufficient level of alkali throughout the impregnation process, avoiding precipitation of lignin on the raw material if spent or semi-spent cooking liquor, i.e. black liquor, is used in impregnation.
In some vessels, depending on type of raw material and cooking process, it could also be preferable that the vessel has means for withdrawing spent impregnation liquids from the other end of the first upper volume of liquid. This reduces the level of dissolved lignin in the subsequent cooking stage, thus promoting further dissolution of lignin in the raw material.
An early withdrawal of impregnation liquid and condensate could also preferably be made at a position in the vessel close to the liquid surface and hence could a large part of the acidic condensate released from the steamed chips be withdrawn, reducing need for charging alkali for neutralization purposes. Such early withdrawal will also reduce harmful content of calcium, which metal is dissolved in acidic conditions and may cause scaling problems in the digester.
An early withdrawal of impregnation liquid at lower temperature also improves the overall heat economy as less mass volumes needs heating in subsequent stages.
One of the primary objects of the present invention is to provide for a simplified continuous digester, with a true single vessel system, having less investment costs as well as less operating costs, but still capable of producing pulp at commercial grades.